Comparing Chlorophyll Meter Readings to Petiole Nitrogen for Potatoes.

The need to determine adequate nitrogen (N) levels for potato crops in Southeast Idaho during the growing season has required expensive petiole sample tests. The use of a SPAD Minolta chlorophyll meter by potato producers could reduce sampling costs and provide accurate in-season potato petiole N levels. The objective of this study is to compare the SPAD chlorophyll meter and petiole samples for accuracy in measuring N petiole levels for in-season N fertilization for potato crops.  Nitrogen treatments of 0, 56, 112, 168, 224, and 280 N kg ha^-1 with three to four replications in a randomized complete block design were used to establish different N levels in the potato crop. Fertilizer was split applied with 56 kg N ha^-1 applied at planting and remainder applied in June and July.  Petiole nitrate (NO₃^-) samples, chlorophyll meter readings, and soil NO₃^- test were taken weekly for 8 wks beginning in July through mid August.  Chlorophyll meter readings were compared to petiole NO₃^- and soil NO₃^- levels, which are current methods for determining in-season N fertilization need.  Potato plots were harvested by digging 3 meters of two adjacent rows and yields were determined based on total weight and weight by quality class.  Yield data were compared with petiole NO₃^-, chlorophyll meter data, and soil NO₃^- levels for determining which method best predicts yield.   Correlation between SPAD meter and petiole showed that in order to meet the early season 20,000 mg kg^-1 petiole NO₃^- threshold the chlorophyll meter reading must meet or exceed 47. For mid season, the petiole NO₃^- threshold drops to 15,000 mg kg^-1 and the correlating chlorophyll reading is 41.  Similarly when the late season petiole NO₃^- threshold drops to 10,000 mg kg-¹ the chlorophyll reading remains 41.  The similar chlorophyll meter reading between the mid and late season petiole NO₃^- levels is due to the lack of new vegetative growth and a lower NO₃^- concentration need in the petioles. Yield data did not correlate to N application, petiole chlorophyll meter readings probably due to large spatial variance and a large amount of residual N.  Our data supports that chlorophyll meter readings accurately indicate N responsiveness of the crop 80-100% of the time when compared to petiole samples. Additional data will be needed to refine and support chlorophyll meter readings as a method to base in-season N fertilization decisions in potatoes.